The use of radar to determine range and velocity of objects in an environment is important in a number of applications including automotive radar and gesture detection. A radar system typically transmits a signal and listens for the reflection of the signal from objects in the environment. By comparing the transmitted signal with the received signal, a radar system can determine the distance to an object. Using multiple transmissions, the velocity of an object can be determined. Using multiple transmitters and receivers, the location (angle) of an object can also be determined.
There are several types of signals used in radar systems. One type of radar signal is known as a frequency modulated continuous waveform (FMCW). In this type of system, the transmitter of the radar system sends a continuous signal in which the frequency of the signal varies. This is sometimes called a chirp radar system. Mixing (multiplying) the reflected wave from a target with a replica of the transmitted signal results in a CW signal with a frequency that represents the distance between the radar transmitter/receiver and the target. By sweeping up in frequency and then down in frequency, the Doppler frequency can also be determined.
Another type of radar signal is known as a phase modulated continuous waveform (PMCW). For this type of signal, the phase of the transmitted signal is changed according to a certain pattern or code, sometimes called the spreading code, known at the radar receiver. The transmitted signal is phase modulated by mixing a baseband signal (e.g. with two values +1 and −1) with a local oscillator to generate a transmitted signal with a phase that is changing corresponding to the baseband signal (e.g., +1 corresponding to a phase of 0 radians and −1 corresponding to a phase of π radians). For a single transmitter, a sequence of phase values that form the code or spreading code that has good autocorrelation properties is required so that ghost targets are minimized. The rate at which the phase is modulated determines the bandwidth of the transmitted signal and is called the chip rate.
In a PMCW radar system, the receiver performs correlations of the received signal with time-delayed versions of the transmitted signal and looks for peaks in the correlation. The time-delay of the transmitted signal that yields peaks in the correlation corresponds to the delay of the transmitted signal when reflected off an object. The distance to the object is found from that delay and the speed of light.
A first radar system receiver may receive a signal from a second radar system transmitter that may be incorrectly interpreted by the radar system receiver as a reflected signal from an object. Such interference in phase-modulated continuous-wave (PMCW) radars is referred to as PMCW-PMCW interference.